`UseWindowsForms` and `UseWPF` properties should be always available when targeting Windows

It's common for libraries and console applications to use miscellaneous APIs from Windows Forms or WPF. In legacy-style projects, you would use "Add Reference" to add `System.Windows.Forms` to your application or library. Today it's not possible to do the same with SDK-style projects without manually editing the project file. Though we do expose `UseWindowsForms` and `UseWPF` in the project properties, they are only visible when the output type is `WinExe` (i.e. not a console application or library).

However, as of .NET 5, we have a more appropriate key for these properties—the target OS. We should change these properties to be visible when `TargetPlatformIdentifier`==`Windows`, rather than when `OutputType`==`WinExe`.